*{
    padding: 0;
    margin: 0;
}

/*外部总容器*/
.container{
    display: flex;
    flex-flow: column;
    align-items: center;
    width: 500px;
    height: 500px;
    margin:  auto;
    border: 2px solid black ;
}

/*显示屏容器*/
.top{
    display: flex;
    justify-content: center;
    align-items: center;
    position: relative;
    top: 10px;
    width: 95%;
    height: 15%;
    border: 2px solid black;
    margin-bottom: 15px;
}

/*显示屏*/
#screen{
    display: flex;
    font-size: 20px;
    width: 98%;
    height: 90%;
    justify-content: flex-end;
    align-items: center;
    border: 2px solid #999999;
}

/*按键区域总容器*/
.center{
    display: flex;
    flex-flow: row wrap;
    justify-content: space-around;
    align-items: center;
    width: 95%;
    height: 78%;
    border: 2px solid black;
}

/*按键容器*/
.btn{
    display: flex;
    justify-content: center;
    align-items: center;
    width: 20%;
    height: 20%;
    border:2px solid black;
}

.btn button{
    font-size: 30px;
    font-weight: 600;
    width: 95%;
    height: 95%;
}